diff --git a/ci/linux-presubmit.sh b/ci/linux-presubmit.sh
index 626989d2..6bac8878 100644
--- a/ci/linux-presubmit.sh
+++ b/ci/linux-presubmit.sh
@@ -78,6 +78,7 @@ time docker run \
       --copt="-Wall" \
       --copt="-Werror" \
       --copt="-Wuninitialized" \
+      --copt="-Wundef" \
       --copt="-Wno-error=pragmas" \
       --distdir="/bazel-distdir" \
       --features=external_include_paths \
@@ -99,6 +100,7 @@ for std in ${STD}; do
         --copt="-Wall" \
         --copt="-Werror" \
         --copt="-Wuninitialized" \
+        --copt="-Wundef" \
         --define="absl=${absl}" \
         --distdir="/bazel-distdir" \
         --features=external_include_paths \
@@ -123,6 +125,7 @@ for std in ${STD}; do
         --copt="-Wall" \
         --copt="-Werror" \
         --copt="-Wuninitialized" \
+        --copt="-Wundef" \
         --define="absl=${absl}" \
         --distdir="/bazel-distdir" \
         --features=external_include_paths \
diff --git a/ci/macos-presubmit.sh b/ci/macos-presubmit.sh
index 8f35df58..681ebc2a 100644
--- a/ci/macos-presubmit.sh
+++ b/ci/macos-presubmit.sh
@@ -66,6 +66,7 @@ for absl in 0 1; do
   ${BAZEL_BIN} test ... \
     --copt="-Wall" \
     --copt="-Werror" \
+    --copt="-Wundef" \
     --cxxopt="-std=c++14" \
     --define="absl=${absl}" \
     --features=external_include_paths \